A Competition Commission investigation is likely to find in favour of the deal, with the £1.51bn takeover set to be signed in December. About £55m of the sale price will be contingent on the number of passengers using the airport.

Michael McGhee, a GIP partner, said that people using the UK's second busiest airport could expect improvements. "We will upgrade and modernise Gatwick airport to transform the experience for both business and leisure passengers.

"We plan to work closely with the airlines to improve performance, as we have done successfully at London City." Mr McGhee said the deal would be funded with a mixture of debt and equity, but debt would make up no more than 50 per cent of the purchase price. Earlier this week, GIP secured a £1.125bn loan from a group of 12 banks, which Mr McGhee says will partly be used to pay for the airport, and partly for capital expenditure at Gatwick.

"We have no exit plans at this stage," he added. "When we do come to sell Gatwick it is likely to be through selling it to another infrastructure fund, or possibly via a public listing."

BAA, which has a huge £9.6bn ($16 billion) debt pile stemming from its 2006 takeover by the Spanish infrastructure group Ferrovial (GRFRF.PK), will use the proceeds to pay down a £1bn bank facility that matures next year. A spokesman stressed that BAA was not forced to sell Gatwick.

However, industry experts said that £1.51bn was a long way short of what BAA had initially wanted for Gatwick.

"BAA said that it was looking for about £2.5bn when it first entered into negotiations about Gatwick, but almost everyone in the industry agreed that was vastly overpriced," said Chris Yates, an aviation analyst. "The price agreed today is about the most the market would take. There is an awful lot of work that GIP now needs to do to bring Gatwick up to world-class standards." GIP declared itself happy with the price.

There were also calls yesterday from union representatives at Unite for immediate talks over jobs at the airport.

BAA was ordered by the Competition Commission to sell Gatwick, as well as Stansted and either Glasgow or Edinburgh airports, in March. The group's three-day appeal against the decision ended yesterday, with a ruling expected early next year.

BAA argues that the original order was "riven" with apparent bias, with a member of the inquiry panel also apparently advising a potential bidder for Gatwick. The group also claims the two-year deadline imposed on BAA to sell the airports is unfair as the recession will dampen any purchase price.